This paper introduces a novel software program for the design of photovoltaic systems called Tlahuillice^TM (Tlahuilli=Light, ce=1 in Nahuatl Language). In comparison to existing programs, it permits an easier access to known algorithms for Photovoltaic System Design, while at the same time being detailed enough for the feasibility & design dimensioning stage for Autonomous or Stand Alone Photovoltaic Systems (Pumping as well as Residential & Building Applications) as well as Utility -Interactive Systems Connected to the Electric Grid. The interface language for Tlahuilli-ce^TM is Spanish, and it is a Microsoft Excel & Microsoft Visual Basic application. Tlahuillice^TM allows people that don't have experience in the solar energy field to use it to achieve good designs.